a) Field of the Invention
The present invention is broadly concerned with a tandem axle suspension for use in a vehicle such as a truck or semi-trailer, having a frame beneath which two axles are mounted in tandem relationship through a set of leaf spring assemblies interconnected by equalizer beams.
The invention is however more particularly yet not restrictively concerned with a tandem axle suspension for use in a vehicle of the above mentioned type, in which the two axles mounted in tandem relationship, support steering wheels and thus are part of the steering system of this vehicle.
b) Brief Description of the Prior Art
Tandem axle suspensions of the above mentioned type in which two axles in tandem relationship are attached to four semi-elliptic leaf-spring assemblies symmetrically attached on both sides of the vehicle and interconnected by means of equalizer beams, are well known in the art and form the subject matter of numerous patents. By way of non-restrictive examples of such patents, reference can be made to the following Canadian and U.S. Pat. Nos.:
______________________________________ Canadian patent Nos.: 279,674 (WARHUS) 1928 527,073 (BOYLES) 1958 705,029 (BOYS) 1965 1,229,860 (JABLE et al) 1987 U.S. Pat. Nos.: 3,378,277 (GADD) 1968 4,033,606 (WARD et al) 1977 4,813,695 (REID) 1989 4,907,814 (FORSTER) 1990 ______________________________________
Basically all these known suspensions comprise the same three major structural elements on each side of the vehicle, namely:
1) a first leaf-spring assembly beneath which the front axle of the tandem is rigidly attached; PA1 2) an equalizer beam rockably mounted about a transversal pivot onto an equalizer support bracket rigidly attached to the frame of the vehicle at a short distance behind the first leaf-spring assembly, this beam being aligned with the first leaf-spring assembly and having a front end extending forwardly with respect to its pivot and and a rear end extending rearwardly with respect to this pivot; PA1 3) a second leaf-spring assembly aligned with the first leaf-spring assembly and extending behind the equalizer beam, beneath which the rear axle of the tandem is rigidly attached. PA1 either to unduly reduce the freedom of the leaf-spring assemblies and thus the efficiency of the same when the ends of these assemblies are connected directly to the frame and equalizer bar, and no sliding is allowed, PA1 or to give too much freedom to the leaf-spring assemblies and thus generate instability problems that can only be solved by the use of stabilization rods also known as "torque rods". PA1 a first leaf-spring assembly having a front end and a read end; PA1 means for pivotably attaching the front end of the first leaf-spring assembly to the frame of the vehicle; PA1 means for rigidly attaching the front axle centrally beneath the first leaf-spring assembly; PA1 an equalizer support bracket rigidly attached to the frame of the vehicle at a short distance behind the rear end of the first leaf-spring; PA1 an equalizer beam rockably mounted onto the equalizer support bracket about a transversal pivot, the equalizer beam being aligned with the first leaf-spring assembly and having a front end extending forwardly with respect to the pivot and a rear end extending rearwardly with respect to this pivot; PA1 a first shackle pivotably attached to the rear end of the first leaf-spring assembly and to the front end of the equalizer beam for operatively interconnecting the same; PA1 a second leaf-spring assembly also having a front end and a rear end, the second leaf-spring assembly being aligned with the first leaf-spring assembly and extending behind the equalizer beam; PA1 means for pivotably attaching the front end of the second leaf-spring assembly to the rear end of the equalizer beam; PA1 means for rigidly attaching the rear axle centrally beneath the second leaf-spring assembly; and PA1 a second shackle pivotably attached to the rear end of the second leaf-spring and to the frame of the vehicle, for operatively interconnecting the same.
So far, the known suspensions essentially rom each other in the way the three above mentioned elements are connected to each other and/or to the frame of the vehicle.
In one conventional embodiment, the front and rear ends of the equalizer beam are pivotaby connected by means of shackles to the rear and front ends of the first and second leaf-spring assemblies, respectively, while the front and rear ends of these assemblies are connected to the frame of the vehicle in such a manner as to be free to move or "slide" longitudinally with respect to it (see U.S. Pat. No. 4,033,606 and Canadian patent No. 1,299,860).
In another conventional embodiment, the rear and front ends of the first and second leaf-spring assemblies are also connected to the ends of the equalizer beam by means of shackles, but the front and rear ends of these assemblies are connected to the frame of the vehicle in such a manner as to be free to pivot but not to slide (see Canadian patent Nos. 279,674; 527,073 and 705,029).
In a further conventional embodiment, the rear and front ends first and second leaf-spring assemblies are pivotably yet not slidably connected to the ends of the equalizer beam while the front and rear ends of these assemblies are connected to the frame of the vehicle in such a manner to be free to slide (see U.S. Pat. Nos. 3,378,277 and 4,907,814) or free to pivot but not to slide (see U.S. Pat. No. 4,813,695).
All of these known suspensions have proved to be generally efficient. However, they all suffer from the same drawbacks which are:
Moreover, on most of the known suspensions where both axles of the tandem are free to slide, another major drawback is encountered. Indeed, due to the way the spring-leaf assemblies are connected, both axles are usually not allowed to move in unison in the same direction when a force is exerted on one or both of them. As a matter of fact, they rather move in opposite directions, thereby applying substantial stress onto rods or bars that may be connected thereto, such as the steering bars of a steering system when the tandem axle is part of such a steering system.
To overcome the latter drawback, it has been suggested to "combine" the three basic structural elements mentioned hereinabove in a completely different manner by connecting the front and rear ends of the equalizer beam to the rear ends of the first and second spring-leaf assemblies, respectively. See, by way of examples, Canadian Pat. No. 250,418 (MARCUM) of 1925 and U.S. Pat. Nos. 2,226,100 (LARSON) of 1939; 3,580,347 (McGEE) of 1971 and 3,933,367 (TAMAS) of 1976. However, such a solution makes the suspension more complicated, and not useful as part of a steering system because of a lack of space on both sides of the vehicle frame.